The technology used in modern energy-saving lighting devices uses mercury as one of the active components. As mercury is harmful to the environment, extensive research is done to overcome the complicated technical difficulties associated with energy-saving, mercury-free lighting .
An approach used for solving this problem is by using a field emission device, such as field emission light source. Field emission is a phenomenon that occurs when an electric field proximate to the surface of an emission material narrows a width of a potential barrier existing at the surface of the emission material. This allows a quantum tunneling effect to occur, whereby electrons cross through the potential barrier and are emitted from the material.
In prior art devices, a cathode is arranged in an evacuated chamber. The chamber may have glass walls. The chamber may be coated on its inside with an electrically conductive layer. A light emitting layer may be deposited on top of the conductive layer. They together constitute an anode. When a potential difference is applied between the cathode and the anode, electrons are emitted from the cathode and accelerated towards the anode. As the electrons strike the light emitting layer, they cause the light emitting layer to emit photons. The process may be referred to as cathodoluminescence. Cathodoluminescence is different from photoluminescence. Photoluminescence is employed in conventional fluorescent lighting devices, such as conventional fluorescent tubes.
Cathodes used in field emission devices are accordingly known as field emission cathodes. Field emission cathodes are considered “cold” cathodes because they do not require the use of a heat source to operate. Among various materials known to be suitable for the construction of field emission cathodes, carbon based materials have proven to be capable of producing significant emission currents over a long lifetime in moderate vacuum environment.
European patent application 99908583, “Field emission cathode fabricated from porous carbon foam material”, discloses a field emission cathode that includes an emission member formed of a porous carbon foam material, such as Reticulated Vitreous Carbon (RVC). The emissive member has an emissive surface defining a multiplicity of emissive edges. RVC is manufactured using a carbonized polymer resin.
The use of RVC as an emissive member has not been completely successful since the material has a period of instability. The period of instability has been referred to as the material's “training period.”,The training period of RVC is believed to result from the desorption of contaminants initially present on the emission surface of the RVC cathode, and by the destruction of the sharpest emissive edges of the RVC material. The latter leads to a complicated fabrication process involving expensive and complex manufacturing steps. Furthermore, the operation voltage of such a field emission cathode, as disclosed above, has to be very high in order to obtain a sufficient output current because too few emission sites over the entire cathode surface.
An object of the present invention to address two crucial issues, the total emission current of the cathode at an appropriate voltage interval, and the uniform spatial and current distributions of the emission edges, in order to provide a novel and improved carbon material for a field emission cathode.